Agricultural tractor hydraulic systems which operate high capacity equipment typically generate a considerable amount of heat which must be dissipated. For example, an implement such as a large seeding tool with a hydraulically driven fan often includes an active hydraulic down force system which operates simultaneously with the fan. Using the tractor selective control valve (SCV) to apply continuous pressure consumes extra engine power and may cause overheating of the tractor hydraulic system. A load sensing system keeps the system pressure at the lowest possible level. Many tractors include an additional output port, referred to as a power beyond supply. The power beyond port provides an external load sense option. However, the power beyond system does not allow the operator to control its output. There is a need to supply a load sensed pressure to the system while allowing the system to be controlled by a non-load sensed control valve.
To prevent load sense pressure from commanding pump flow during tractor engine startup, thus creating tractor starting issues under certain circumstances, it is necessary to prevent load sense signal pressure from being communicated to the tractor during engine startup. This could be done with an electric solenoid valve but an electrical signal would have to be present and made available to the circuit. A method to accomplish activation and deactivation of the down force is needed that does not require an electrical signal. Since not all tractors are equipped with power beyond, it is also beneficial to be able to provide down force pressure to the wing cylinders using an SCV connection.
The sequencing of the wing lift and the center frame lift as well as sequencing of the center frame lowering and the wing lowering are crucial. The wings must lift before the center frame lifts and must stay lifted until after the center frame is lowered. Adding continuous back pressure to the wing cylinders through an active down force system creates a challenge for proper sequencing.
The former method of sequencing the wing and center frame raise and lower functions uses cylinder size and atmospheric pressure to achieve the correct sequencing. When the SCV is actuated to raise the center frame, the pressure required to retract the wing cylinders is less than the pressure required to extend the center frame cylinders. Therefore, the wings lift before the center frame is raised. When the SCV is actuated to lower the center frame, the back pressure caused by oil returning from the center frame cylinders is enough to keep the wing cylinders retracted. When the center frame cylinders fully retract, the flow drops and pressure drops, allowing the wing cylinders to extend.
A partial solution to the above problems is provided in US Patent Publication US2010/0078185, commonly assigned with the present application and hereby incorporated by reference. There, additional valve components are configured into the active down force circuit to cause the tractor hydraulic system to operate below the stall or high pressure standby condition. A check valve connects the tractor power beyond supply line to the pressure reduction valve that is connected to the implement cylinder ends and controls down pressure. The tractor selective control valve is then operated at load pressure in the float mode when the down force circuit is controlling implement down pressure. The circuit eliminates a stall signal to the hydraulic pump that otherwise would cause the pump to rise to the high, heat-producing stall pressure when operating in the active pressure mode. During implement lift, a check valve allows hydraulic flow from the cylinders to bypass the pressure reduction valve. The system therefore operates at lower pressure and lower power to produce less heat and increase fuel economy. The above patent application does not provide for sequencing other than by cylinder sizing.